FM radio frequency (“RF”) signals have been traditionally broadcast in an analog format, and therefore transmission systems have largely remained unchanged. See FIG. 1. Recently, the Federal Communications Commission (“FCC”) has accepted a new digital format known as In-Band-On-Channel (“IBOC”), as a method of transmitting digital information on FM radio signals. IBOC requires the simultaneous broadcast of an analog and a digital signal within one channel of the FM band. By utilizing digital subcarriers, see, e.g., FIG. 2, digital information is placed on both sides of the FM analog signal allowing IBOC transmission of the digital information, and alleviating the complications of requiring a separate channel allocated to the digital signal, during the period of transition from analog radio to digital radio. Accordingly, IBOC is a hybrid mode of transmission, where both analog and digital signals are broadcast together on a single channel, and eventually, the analog FM will be dropped for the all-digital signal with room for more information.
The strategy of combining separate analog and digital signals involves at least two important requirements. The first requirement is to design a filter that meets the FCC mask requirements for the combined digital and analog signal. The second requirement is to combine the two signals into one antenna without degrading either signal. Currently, digital sidebands are 1/100th, or −20 dB, of the power in the analog signal, which often results in poor reception of the IBOC signal. Therefor, it may be necessary in the future to increase the digital power another 10 dB, or 1/10th the power of the analog signal.
There are currently four different strategies proposed for transmitting an analog signal and a digital signal together.
The methods of combining the digital and analog signals consist of:
1) Common Amplification: A low power level combiner is used before the signal is amplified. This method becomes difficult to implement in high power transmitters due to the high peak to average ratio of the digital signal. The transmitters cannot handle the high peak powers.
2) High Level Combining: A high power combining system where a directional coupler is used to inject the digital signal. The analog signal is run through the main line of the coupler while the digital signal is input to the coupled line. The coupling value is typically −10 dB resulting in significant losses (90%) to the digital signal and therefore requires a transmitter with 10 dB more power than required. The lost power is dumped into a dumpy load where it is dissipated into heat. There is also 10% loss of the analog signal which requires existing transmitters to increase power to maintain coverage. Most high power analog transmitters do not have this kind of headroom.
3) Mid Level Combining: A high power combining system where a directional coupler is also used to inject the digital signal. This differs from high level combining in that the digital transmitter has some common amplification (analog and digital) allowing reduced coupling values thereby minimizing digital and analog losses. In mid level combining, the digital losses exceed 3-6 dB. See U.S. Published Patent Application No. 20050190851, to Cabrera.
4) The use of separate antennas or orthogonally fed antennas. The obvious drawback to this strategy is the cost of the duplicative antenna and the transmission equipment needed.
In the field of terrestrial FM broadcast, high power RF filters are used for several applications. Generally, high power filters; bandpass, notch, lowpass, and highpass, are used to attenuate unwanted signals or to combine several stations to one transmission line or to combine two or more different channels to one transmission line. Unwanted signals include out-of-band emissions generated at the transmitter and signals coming back down the antenna. RF filters used for channel combining applications are designed to ensure low losses for each channel being combined.
Channel combiners take the form of junction combiners, e.g., tar point and manifold (FIG. 3) and directional filter modules. See, e.g., FIG. 4. Referring to FIG. 3, junction combiners can either be “manifold” or “starpoint” combiners. In both cases, each channel passes through its own filter and proceeds toward the junction. When the signal reaches the junction, it is split and delivered toward other filters. When the split signals reach the other channel filters, they are fully reflected and returned toward the junction. The line lengths between other filters and junctions are phased, or, separated in distance, so that they re-combine and are sent to the output.
Similarly, high power FM directional combiners are used to combine separate high power FM channels for transmission by one antenna. See, e.g., FIG. 5. Typically, 3 and 4-section filters are used. Quadrature hybrids are used to split and recombine coherent signals. Referring to FIG. 4, Channel A is split in the first hybrid and passes through the filters. The signal is then recombined in the output hybrid and directed to the output. Channel B is delivered to the output hybrid, split and then reflected off the filters which are tuned to pass only Channel A. The filters provide enough rejection (typically about 30 dB), to fully reflect Channel B, where Channel B is recombined and sent to the output port with Channel A. The load is sized to absorb reflected power from Channel A input due to filter reflection coefficient and any power from input Channel B that passes through the filters due to imperfect rejection.
It has been postulated that the high power FM directional combiner could be used to combine an analog FM signal with digital sidebands within the same channel (FIG. 5). Plonka combined adjacent TV channels using a directional combiner with “sharp tuned” filters. See U.S. Pat. No. 7,224,400 to Plonka. Referring to FIG. 6, which is a spectrum diagram of an IBOC signal at a specific frequency, the bandpass filter needs to straddle the FM carrier frequency by +/−0.075 MHz because this is where most of the FM power is. In order to do this, the filter rejection would need to be approximately −30 dB at carrier +/−0.13 MHz, where the digital sidebands are (+/−0.1 MHz for an extended mode). This results in a narrow filter with exceptionally high loss on the analog signal. An example of the sharp tuned filter response, or filter rejection, is shown in FIG. 7. The −30 dB rejection requirement is for low loss digital sideband combining. When this filter rejection requirement is met, the resulting combiner losses to the analog FM signal is unacceptable. Referring to FIG. 8, the resulting FM analog loss using a sharp tuned filter is seen to be about 1.4 dB mid-band, with 0.9 dB loss variation and 1.7 dB integrated loss over carrier +/−0.075 MHz. The integrated digital sideband loss, however, is 0.57 dB with 1.25 dB loss variation.
Thus, while the digital loss for a sharp tuned filter was better than previous systems, i.e., 0.57 dB vs. 3 to 6 dB, the analog FM loss was unworkable. That is, the loss variation is uncorrectable because there is too much delay and amplitude variation. Also, the mid-band loss and the integrated loss over carrier is too large. To compensate for these losses, the signal would have to be excessively amplified, and the existing transmitters do not have the head room to provide this amplification. Moreover, the 8-section filters that would be required make the size of the combiner itself much too large and run too hot due to dissipated losses.
Accordingly, a need exists for a high power combiner to combine an FM analog signal with digital sidebands within the same channel without excessive loss of signal, such that the combiner losses would be acceptable without the need for an oversized, and expensive, combiner.
The present invention provides a solution to the above problems by providing a high power FM combiner comprising a “mild tuned” filter. Applicants have surprisingly found that by using a mild tuned filter, heretofor unknown in the art for combining an analog FM signal and digital sidebands in the same channel, results in an IBOC signal with moderate but acceptable losses in both the analog and digital signals. The analog losses can be corrected by amplification in a range that can be accommodated by existing transmitters with sufficient headroom. Similarly, the loss and delay variations are correctable by methods know to those of skill in the art. Further, the within invention obviates the need for 8-section filters, because 4-sections filters are sufficient. Such 4-section filters are both smaller and less costly.
In addition to the above features, the invention may be implemented easily without adding significant cost or complexity above the current high power FM combiners, and avoids the need for more than one antenna for transmission.